 Ptolemae's ideas of the geocentric universe and the complicated system of epicycles that were needed to keep the planets in their retrograde motions around the earth would dominate Western cultural thinking for at least 1,500 years, well into the 17th century Europe. However, mathematicians such as Nicholas Copernicus and other astronomers began to wonder about the heliocentric model that was first proposed many thousand years previous. The reason for this was that Ptolemae's epicycles were starting to break down again. They were becoming less accurate at predicting the upcoming retrogrades and moreover, they were messy and they were complex. But Nicholas Copernicus would become the first astronomer to publish a mathematical explanation of retrograde motion with the Sun at the center. Now Copernicus believed that the planets still moved in perfectly circular orbits and they also had every reason to believe that the planets moved at constant speed at the same time. So if we have earth here with Mars, you can imagine looking out toward Mars against the background stars, Mars would appear here. But over time as earth and Mars would move along, Mars would seem to move in prograde motion and like a runner on an inside track overtaking an outer moving slower object, the planet would appear to backtrack or execute a retrograde loop in the stars before resuming its path around the Sun again. So this was a mathematical way to reproduce retrograde motion while keeping the Sun at the center of the solar system. So retrograde motion did not need an epicycle, it did not require a physical backward motion of the planet. Rather it was simply an apparent motion of the planet when earth overtakes an outer planet known as a superior planet or when earth is overtaken by an inferior planet, that is to say an inner planet. And this was much simpler than epicycles. Now this was not by any means a perfect explanation. Copernicus' ideas still relied on things like circular orbits and constant speeds for the planets, but nevertheless his ideas of the heliocentric model of retrograde motion would start to peak the interest of many other thinkers at that time. By the early 1600s, Galileo Galilei had arrived on the scene and had really become what we now think of as the first modern scientist, and that is he would take the claims that were taught largely by the governing body at the time, the Holy Roman Catholic Church, and tested those claims. He simply conducted experiments, which is something that modern scientists do to this day. However, his results would contradict many of the Church's teachings and for his troubles he would be placed under house arrest where he would live the rest of his days until recanting prior to his death. However, to be fair, the Catholic Church eventually did apologize for placing him under house arrest in the early 1600s in 1992. That's when Pope John Paul apologized on behalf of the Church. Galileo is widely thought of as the first astronomer to ever use a telescope, but that's really more of a testament to Galileo's ability to market and promote himself. In fact, he was not the first person to use a telescope against the heavens, but he did make very detailed observations. And these observations would contradict the Church's teachings. For example, his observations of the moon would show that the moon, in addition to going through phases, was not perfectly smooth. It was jagged and cratered, and this was in contradiction to the Church's teachings, that the moon was perfectly spherical and perfect and translucent and immutable. It was not immutable, it was in fact highly mutated. It had been banged up a few times, and he discovered these craters in great detail. Another observation that he made was Venus. It turns out that Venus goes through phases like the moon, but unlike the moon, Venus would appear to change size. It would always be at crescent when it was largest in his telescope, and it would always appear full when it was smallest in his telescope. And the only way to get Venus to make these phases while at the same time changing its sizes is to assume that Venus is going around the Sun and that Venus is closer to the Sun than the Earth. So this observation was in direct contradiction to the geocentric model that was taught by the Church. His observations of Jupiter's moons revealed the presence of four tiny little stars that he named the Medician stars after the Grand Duke of Tuscany. It was he who was funding Galileo, so Galileo wanting to do right by his benefactor named these stars after him. Well, these stars are in fact four of Jupiter's largest moons, and to this day we refer to them as the Galilean moons of Jupiter, Io, Europa, Ganymede, and Callisto. In any event, Galileo noticed that from one night to the next, the positions of these moons would change, and he correctly concluded that these moons are in fact going around Jupiter much like our moon goes around the Earth. Now this did not directly prove that the Sun was at the center of the universe. However, what it did do was stand in stark contradiction to the Church's teaching that everything orbits Earth. Well, Galileo argued, if everything orbits the Earth, what are these things doing orbiting Jupiter? So this was another direct violation of the geocentric model. Now it's also widely rumored that Galileo disproved the idea that heavier objects must fall faster than lighter objects by dropping rocks of different weights off of the Leaning Tower of Pisa. He almost certainly did not do that. It's really, again, I think just one of Galileo's brilliant attempts at marketing. Nevertheless, he did arrive at a conclusion that was in contradiction to the Church's teachings. Objects would not fall according to their weight. They all fell at the same speed regardless of their weight, and this was famously tested in a demonstration during- Uh, can we copy to both solar wind and a counter drum in the ETB? Quite, yes. I haven't put the solar wind in yet, but I will shortly. Oh, I'll watch this. Joe, do you have a good picture there? I've got the- Beautiful picture, Dave. My left hand, I have a feather. And my right hand, a hammer. And I guess one of the reasons we got here today was because of a gentleman named Galileo a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought that where would be a better place to confirm his findings and on the moon. And so we thought we'd try it here for you. And the feather happens to be appropriately a falcon feather for our falcon. And I'll drop the two of them here, and hopefully they'll hit the ground at the same time. How about that? How about that? It proves that Mr. Galileo was correct in his findings. Superb. Unfortunately, the church did not take too kindly to Galileo's evidence and placed him under house arrest for heresy. He would remain there until just before his death when he recanted, taking back everything he ever said. Turns out, though, he was right.